This invention relates to fuel burner control systems which utilize an electrical resistance igniter.
Fuel burner control systems with hot surface ignition, wherein a main burner is directly ignited by an electrical resistance igniter, are becoming more widely used. While the prior art discloses various such systems which appear to perform adequately, there is a need to improve the overall performance and reliability of the electrical resistance igniter used therein.
Specifically, the typical electrical resistance igniters utilized in prior art systems generally require between approximately 15 and 45 seconds of electrical energizing to warm up to a temperature sufficiently high to ignite the air-fuel mixture at the burner. While such warm-up times present no particular performance problem, they are a disadvantage with regard to testing the system on the assembly line of the device incorporating the system. Specifically, in the assembly line of the device such as a furnace or boiler utilizing the system, the system is tested to determine that it operates properly. Among tests performed is a test to determine that the igniter does, in fact, attain a temperature sufficiently high to ignite the air-fuel mixture. Therefore, unless normal system function is bypassed or altered in some manner for this test, the igniter will be energized for a time period somewhere between 15 and 45 seconds. Since a test time of 45 seconds, and to a lesser extent, a test time of 15 seconds, are significant cost factors, particularly in a high-volume assembly line, it is desirable to provide an igniter with faster warm-up time so as to reduce such test times.
Additionally, there are various devices which provide a heat output within a very short time, such as less than 10 seconds, after a demand for heat is initiated. Such devices generally utilize spark ignition. It is desirable to provide an electrical resistance igniter having a sufficiently short warm-up time to enable such an igniter to be used in lieu of spark ignition in such devices.
It has been determined that a warm-up time shorter than that in the prior art systems is attainable with an electrical resistance igniter constructed of a tungsten heater element embedded in a silicon nitride insulator. While such an igniter, hereinafter referred to as a silicon nitride igniter, appears to possess the inherent capability of providing the desired feature of a fast warm-up time, it has certain characteristics which necessitate the use of unique control system circuitry.
Specifically, the silicon nitride igniter has a relatively narrow useable temperature range. That is to say, the temperature span between the lowest ignition temperature which will effect ignition and the highest temperature which the igniter can safely and reliably withstand is relatively narrow. If the igniter is repeatedly energized so that its temperature is at or near such a highest temperature, the igniter will eventually fail, such failure generally consisting of melting of the tungsten heater element.